The sewing process is an important step of the production of garments. Basically, the sewing process involves fastening of fabrics, or other materials, with the help of a needle and threads. During the sewing process, needles of sewing machines, or overlock machines, are subjected to an increment of temperature due to friction between the needle and the portions of the garment that are being sewn together. For high speed sewing in overlock machines used in textile industry, needles may overheat up to high temperatures. In many circumstances, in correspondence of the sewing needle tip or needle eye, temperatures as high as 300° C. have been measured. Such high temperatures during the stitching process may damage both the textile material of the garment and the sewing needle. Moreover, on the edge of the loops, portions of the stitch and of the fabric go into melting, resulting in critical effects for both functionality and aesthetic appearance of the seam and the entire garment.
The problem of damaging the fabric material of the garment during sewing process with overheated needles is even major in the case of synthetic fibres such as spandex, lycra or other polyester-polyurethane copolymers. In fact, in such cases, the synthetic textile materials are torn or melted and covered by the sewing thread, resulting weaker in correspondence of the seam. In many cases, part of the textile material while melting sticks on the needle, causing stitching damages, torn threads and needle deterioration.
Heat is naturally dispersed from the overheated needle through spontaneous heat dispersion such as radiation, convection and conduction.
By means of thermal radiation, an object with a non-null temperature, such as the needle in the case of sewing processes, emits a radiation that result in an output thermal power. However, radiation plays a relatively small role in the needle cooling during sewing operations.
Thermal convection is a heat transfer generated by the movement of fluids that surround an object. In the case of the sewing needles, convection heat transfer is caused by the needles movement in air during the oscillating movement of needles during sewing operations.
Thermal conduction is a heat transfer generated by diffusion and microscopic collision of particles within a body or between contiguous bodies. In the case of sewing needles, it represents the most important heat dispersion component that induces needles decreasing its temperature when the machine stops. The heat flows by conduction from the overheated needle to the retaining means of the needle, to the arm of the sewing machine and to the entire sewing machine.
However, spontaneous heat dispersion through radiation, convection and conduction as described above may result not sufficient for eliminating the undesired effect of needle heating during sewing operations and the possible correlated damages caused to textile fabrics, seams and needle itself.
Document U.S. Pat. No. 2,690,148 discloses a device for cooling an overheated sewing machine needle during sewing operations. More in detail, it discloses a needle cooler comprising a hollow C-shaped body that has an inlet communicating with a flexible hose that in turn communicates with a supply of cooling medium under pressure. The C-shaped body is connected to the clamp screw that retains the needle in a fixed position during sewing operations. The lower part of the C-shaped body is a hollow chamber with a series of spaced jet outlet passages inclined so that they converge upon a common point at which the eye of the needle is located. The cooling effect of the needle is obtained by convection performed by turbulent jet streams.
However, the structure of the cooling device is complex and the cooling effect obtained by turbulent jet streams may imply the interaction of jet streams with the fabric materials that is positioned close to the eye of the needle where the jet streams converge.
Document U.S. Pat. No. 2,316,647 discloses a needle cooling device for sewing machines comprising a coolant reservoir and liquid dispensing means so that liquid coolant is applied to the needle at each reciprocation. In particular, the cooling device is provided with a container comprising an absorbent fibre able to carry the desired amount of liquid coolant. When the needle descends it will pass through the fibre and will pick up from the fibre the liquid that will cause the cooling of the needle. In this way, the needle tip and the needle eye are subject to a decrease in temperature. The cooling effect of the needle is obtained by convection performed by the liquid coolant and by conduction, performed by the contact between the sewing needle and the absorbent fibre with the liquid coolant.
However, the cooling effect is not continuous and during its movement, the needle carries the liquid coolant picked up from the absorbent fibre of the cooling device. In this way, during sewing operations, the liquid coolant can interact with the fabric material when the needle passes through the portions of the garment.
As a result, the known methods for cooling sewing needles disclose devices that use cooling fluids for cooling overheated needles during sewing operations.
According to the known art, the cooling fluids interact with the needle, causing the needle to decrease its temperature, but the cooling fluids interact with the fabric material too. This is a drawback of the know art, since cooling fluids provide invasive effects that can damage the fabric. In fact, many fabric materials are temperature sensitive and cooling fluids on the fabric may result in a deterioration of the portion in contact with the cooling fluids. Additionally, the known cooling system for sewing machine, especially those using cooling fluids, are difficult and expensive to maintain. In particular, the known cooling system requires high maintenance time and costs.